Gradation thermal printhead and gradation heat transfer printing apparatus

ABSTRACT

A gradation thermal printhead having a plurality of heat generating elements each having a heat generating body connected between a pair of electrodes, the electric resistance value of the heat generating body being locally increased in one or more portions thereof. Disclosed also is a gradation heat transfer printing apparatus making use of the gradation thermal printhead mentioned above and employing an energy controlling means for varying the voltage or pulse width of the signal pulse voltage applied to the thermal printhead, thereby allowing a control of the area of the printed dot corresponding to one heat generating element, i.e., the density of printing, thus attaining a gradation control of the printed image.

This application is a continuation, of application Ser. No. 729,039,filed Apr. 30, 1985, now abandoned.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a thermal printhead and a thermaltransfer printing apparatus.

(2) Description of the Prior Art

Hitherto, attempts have been made for effecting a gradation printing bythermal transfer printing method by making use of a thermal printhead.It is, however, extremely difficult to change the printing density ofeach dot formed by each heat generating element. Conventionally,therefore, the gradation control has been made in a false manner byusing a plurality of dots in representing a single picture element. Thismethod inconveniently complicates the signal processing and reduces theresolution of the printed image as compared with the density ofarrangement of the heat generating elements on the recording head, thusimpairing the fineness of the printed picture.

This problem of the prior art will be explained hereinunder withreference to the accompanying drawings.

FIG. 1 schematically shows the construction of a thermal transferprinter. The printer has a thermal printhead 1 having a plurality ofheat generating elements 1R arranged in a direction perpendicular to theplane of the sheet of the Figure. A reference numeral 2 designates athermal transfer ink sheet which is applied to the base film 2a by meansof a heat-meltable ink 2b. As shown in the Figure, the thermal transferink sheet 2 and a printing paper 3 are adapted to run in the directionof an arrow, through the gap formed between the thermal printhead 1 anda platen 4. In order to ensure a close contact between the printingpaper 3 and the thermal transfer ink sheet 2, as well as between thethermal transfer ink sheet 2 and the heat generating elements 1R of thethermal printhead 1, the platen is urged towards the thermal printhead1.

FIG. 2 shows the detail of the array of the heat generating elements 1aon conventional thermal printhead 1. As will be seen from this Figure,each heat generating element 1R has a heat generating body 1b connectedbetween a pair of electrodes 1a, 1a. In operation, voltages are appliedselectively to the heat generating elements 1R in accordance wtih therecording signal, so that the selected heat generating elements 1Rgenerate heat. As a result, the heat-meltable ink 2b on the portions ofthe thermal transfer ink sheet 2 adjacent the selected heat generatingelements is molten and transferred, thus printing the data correspondingto the recording signal.

FIG. 3 shows the relationship between the amount of heat applied to thethermal transfer ink sheet 2 and the density D of the image formed onthe printing paper 3.

As will be seen from FIG. 3, the printing density D exhibits anextremely large gradient when the heat amount Q ranges between Q₁ andQ₂, but the curve representing the density D is flat in the regionbetween Q₂ and Q₃.

FIG. 4 shows the states of printing of dots on the printing paper 3corresponding to the heat amounts Q₁, Q₂ and Q₃, respectively. Theprinting cannot be made satisfactorily when the amount of heat fallsbelow Q₂, due to failure in the transfer of the heat-meltable ink 2b tothe printing paper 3.

Then, as the amount of heat is increased to Q₂, the heat-meltable ink 2bin the whole area corresponding to the heat generating element 1R ismolten and transferred to the printing paper 3, thus drasticallyincreasing the printing density. However, when the heat amount isincreased beyond Q₂ up to Q₃, the printing density is not increasedappreciably and adjacent dots merge into each other undesirably.

Thus, the heat-meltable ink of the thermal transfer ink sheet 2 is notmolten sufficiently and, hence, the transfer to the printing paper isnot attained satisfactorily, unless the heat applied by the heatgenerating element exceeds a predetermined level. In other words, thetransfer is made or not made depending on whether the heat input exceedsor not a predetermined threshold region which is between Q₁ and Q₂ inthe case of the example shown in FIG. 3.

Therefore, it has been difficult to attain a smooth gradation control bylinearly changing the heat amount applied to the thermal transfer inksheet 2, when the printing is conducted on the thermal transfer inksheet 2 by means of the conventional thermal printhead.

For this reason, when it is necessary to effect a gradation printing byusing a thermal printhead having heat generating elements eachconsisting of a heat generating body 1b connected between a pair ofelectrodes 1a, 1a as shown in FIG. 2, it has been a common measure touse a matrix of a plurality of heat generating elements for representingeach picture element and to change the number of dots, i.e., the heatgenerating elements, taking part in the heating operation in eachmatrix, thus attaining a gradation control by controlling the areaoccupied by dots as shown in FIG. 5.

More specifically, FIG. 5 exemplarily shows the case where each pictureelement is constituted by four dots, with each square representing theprinting area for one dot.

When the gradation printing is conducted in accordance with theexplained method by making use of the conventional thermal printheadshown in FIG. 2, the density of the picture elements, i.e., theresolution, is undesirably lowered as compared with the density ofarrangement of the heat generating elements on the thermal printhead, sothat the fineness of the print is impaired and a complicated procedureis required for processing the signals.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide, by acomparatively simple way, a gradation thermal printhead and a thermaltransfer gradation printing apparatus capable of making a sufficientgradation control and attaining a print image of a comparatively highresolution, thereby overcoming the above-described problems of the priorart.

To this end, according to one aspect of the invention, there is provideda thermal printhead having a plurality of heat generating elements eachhaving a heat generating body disposed between a pair of electrodes,wherein the heat generating body is provided with one or more portionsexhibiting high electric resistance.

According to another aspect of the invention, there is provided athermal transfer gradation printing apparatus having the thermalprinthead of the invention explained before and an energy control meansfor controlling the energy applied to the thermal printhead throughcontrolling the voltages applied to the heat generating elements or thetime durations of application of voltages to these heat generatingelements. According to this arrangement, it is possible to freely changethe area of the printing dot for each heat generating element, i.e., theprinting density, so that the gradation control can be made by way ofeach dot, thus eliminating the necessity for the use of a plurality ofdots for single picture element. Consequently, the printing can be madewith a well controlled gradation and at a high resolution correspondingto the density of arrangement of the heat generating elements on thethermal printhead, thus allowing a high degree of fineness and gradationof the print.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a conventional thermal transferprinter;

FIG. 2 is a plan view showing the construction of a conventional thermalprinthead;

FIG. 3 is a chart showing the relationship between the amount of heatapplied to the thermal transfer ink sheet and the printing density;

FIG. 4 is an illustration of the states of printing of dots on aprinting paper, corresponding to heat input amounts Q₁, Q₂ and Q₃ ;

FIG. 5 is an illustration of an example of printing by a conventionalmethod in which a single picture element is given by four dots so thatthe gradation of each picture element is controlled by way of area ofthe dots;

FIGS. 6A, 6B and 6C are illustrations of an embodiment of the gradationthermal printhead in accordance with the invention;

FIG. 7 is a characteristic diagram showing the distribution ofresistance values along the length of each heat generating bodyincorporated in the thermal printhead shown in FIG. 6A.

FIG. 8 is a characteristic diagram showing the distribution ofresistance values along the length of the heat generating element shownin FIG. 6C;

FIG. 9 is an illustration of the distribution of amount of heatgenerated by the heat generating element and the state of recording ofeach dot as observed when the time duration of application of voltage tothe heat-generating element shown in FIGS. 6A to 6C is varied;

FIG. 10 is an example of a picture printed by the gradation printingconducted in accordance with a first embodiment of the invention;

FIGS. 11, 12 and 13A are illustrations of other examples ofheat-generating elements for use in the gradation thermal printhead inaccordance with the invention;

FIG. 13B is an illustration of the state of printing conducted by usingthe thermal printhead in accordance with the invention shown in FIG.13A; and

FIG. 14 is an illustration of a thermal transfer gradation printingapparatus in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6A shows a first embodiment of a gradation thermal printhead inaccordance with the invention. This thermal printhead has a plurality ofheat generating elements 7 arranged along a line, each heat generatingelement having a heat generating body 6 connected between a pair ofelectrodes 5a and 5b. The time duration of application of voltage to thegradation thermal printhead is controlled by an energy controlling meanswhich is incorporated by a thermal transfer gradation printing apparatusprovided in accordance with another aspect of the invention. Aheat-meltable ink on the heat transfer ink sheet is transferred by theheat generated by the heat generating elements supplied with thevoltage.

More specifically, in FIG. 6A, reference numeral 1 denotes a heatgenerating substrate, 8 denotes a base, 9 denotes a semiconductorelement for driving the heat generating body, 10 denotes a semiconductorelement protective cover and 11 denotes a lead wire.

According to the invention, a plurality of the heat generating bodies 6are arranged in a row on the heat generating substrate 1. The heatgenerating bodies 6 are connected at their one ends to a commonelectrode 5a, while other ends are connected to independent picturesignal electrodes 5b. As will be seen from FIG. 6B, the common electrode5a continues to the reverse side of the heat generating substrate 1.Driving electrodes 8c for driving the semiconductor elements 9 fordriving the heat generating bodies are formed on the same side of theheat generating bodies as the picture signal electrodes 5b. Each drivingelectrode 8c is composed of a pattern portion 8c1 which is wire-bondedto the semiconductor element 9 and a connecting terminal portion 8c2 forexternal connection. The picture signal electrode 5b is connected at itsone end to the heat generating body 6 while the other end is wire-bondedto the semiconductor element 9. A heat generating element substrate 8 isbonded to the base 1 by means of a bond through the intermediary of aninsulating layer of polyimide. The substrate 1 is provided with a notch1a through which the lead wire 11 is extended externally. As will beseen from FIG. 6B, the lead wire 11 is fixed by soldering to the commonelectrode 5a spreading on the reverse side of the heat generatingelement substrate 8. The semiconductor protective cover 10 is secured atits both sides to the corresponding sides of the heat generating elementsubstrate 8 by means of both-sided adhesive tapes so as to form a gapbetween itself and the heat generating element driving semiconductorelement 9 thereby protecting the latter. In this case, the space betweenthe semiconductor element 9 and the protective cover 10 is filled with aheat conductive compound agent so that the protective cover 10 isutilized as a heat radiation plate. The common electrode 5a is formedover one end face of the substrate 8 and spread over the reverse side ofthe substrate 8 so that it has a large width to reduce the wiringresistance to a negligibly small level.

FIG. 6C is an enlarged view of a part of the embodiment shown in FIG.6B, showing in particular the portion around the heat generating bodies6. As will be seen from this Figure, each heat generating body 6 has awidth which is smallest at the central portion thereof and graduallyincreased towards the ends connected to respective electrodes 5a and 5b.Therefore, the heat-generating body exhibits such a distribution ofresistance value R in the longitudinal direction X as having a peakvalue at the central portion and gradually decreases towards both endsconnected to the electrodes 5a and 5b. FIG. 8 shows the distribution ofthe amount of heat Q generated in the heat generating body 6 in thedirection of arrow X when a given voltage is applied between bothelectrodes 5a and 5b. It will be seen that the rate of generation ofheat is greater at the central portion of the heat generating body 6where the resistance value is greatest.

FIG. 14 shows an example of the thermal transfer gradation printingapparatus in accordance with the invention. This printing apparatusincorporates a plurality of gradation thermal printheads of the firstembodiment of the first aspect of the invention explained before inconnection with FIG. 6A. A plurality of heat generating bodies 7 arearranged in the direction perpendicular to the plane of sheet of thedrawing. The printing apparatus incorporates a thermal transfer inksheet 22 which is equivalent to that of the conventional apparatusexplained before in connection with FIG. 1. The thermal transfer inksheet 2 is paid-off from a pay-off roll 22a and taken up by a take-upreel 22b. A reference numeral 23 designates an image receiving papersupplied from a roll 23a, while 24 denotes a platen which opposes thegradation thermal printhead 21 across the image receiving paper 23 andthe heat transfer ink sheet 22. During the printing, the platen 24 isrotated in the direction of the arrow so that the printing is madesuccessively on the image receiving paper which is successively fed fromthe roll 23a. The printing apparatus further has a rotary cutter 25which is adapted to automatically cut the image receiving paper 23ejected after the printing. The printing apparatus further has a powersupply 26 for the gradation thermal printhead and an energy controllingmeans for controlling the width of the pulse of the voltage applied tothe thermal printhead in accordance with the recording signal.

FIG. 9 shows the distribution of amount of heat Q generated in the heatgenerating element 7 shown in FIG. 6A in the direction X, as observedwhen a given voltage is applied between both electrodes 5a and 5b by theprinting apparatus shown in FIG. 14, while varying the time duration ofapplication of the voltage from t1 to t4 by the operation of the energycontrolling means 27. FIG. 9 shows also the states D1 to D4 of printingof dots obtained on the printing paper corresponding to the distributionof the heat Q. In FIG. 9, the level QM of the heat Q represents the heatamount which is capable of melting the heat-meltable ink 22 and, hence,capable of transferring the ink to the recording paper 3. It will beseen that the ink is molten and, therefore, transferred to the printingpaper, only at the portion of the thermal transfer ink sheetcorresponding to the portion of each heat generating element 6 whichproduces the heat Q in excess of the level QM. Therefore, the dotdiameter is increased as the time duration of application of the signalvoltage is increased, as will be understood from the comparison of thedots D1 to D4 shown in FIG. 9. Needless to say, the greater dot diameterprovides a higher printing density. According to the invention, it isthus possible to effect the area gradation by way of each dot, bycontrolling the time duration of application of the signal voltage.

From the foregoing description, it will be understood that the gradationthermal printhead and the gradation thermal transfer printing apparatusof the invention does not require the use of a plurality of dots forforming a signal picture element, so that the signal processing isconducted easily and the number of drivers for applying voltage to theheat generating elements can be reduced. In addition, a high resolutioncorresponding to the density of arrangement of the heat generatingelements 7 on the gradation thermal printhead is obtained, as well as afine control of the gradation.

FIG. 10 shows an example of an image formed by gradation printingconducted by a gradation thermal transfer printing apparatus of thesecond aspect of the invention employing the gradation thermal printheadin accordance with the first embodiment of the first aspect of theinvention. It will be seen that the resolution and gradation are muchimproved so that the image quality is much better than the image shownin FIG. 5 produced by the conventional thermal printhead.

FIGS. 11 and 12 show other examples of the heat-generating element usedin the gradation thermal printhead of the invention. It will be seenthat these examples have forms of the heat generating body 6 betweenboth electrodes 5a and 5b different from that shown in FIG. 6A, althoughthese examples produce substantially equal effect as that produced bythe arrangement shown in FIG. 6A. The examples shown in FIGS. 11 and 12offer advantages in that the fabrication is facilitated and the cost isreduced by virture of the simple form of the heat generating bodiescomposed of straight lines.

FIG. 13A shows a further example of the heat-generating element used inthe gradation thermal printhead of the invention. In this case, the heatgenerating member 6 connected between the pair of electrodes 5a and 5bis provided at 6 (six) portions thereof with notches and apertures A sothat four regions encircled by circles B exhibit greater resistance and,hence, a greater heat generation when the signal voltage is appliedbetween the electrodes 5a and 5b. FIG. 13B shows the state of dot asobtained when the printing is conducted by the gradation heat transferprinting apparatus of the invention incorporating the gradation thermalprinthead of the invention which employs the heat generating elementsshown in FIG. 13A, while varying the width of the signal voltage pulseapplied to the heat generating element. In this case, the dot formed byone heat generating element is constituted by four fine dot segmentscorresponding to the aforementioned regions B and the area of each dotsegment is gradually increased as the pulse width is increased, thusattaining a gradation printing. This embodiment provides the bestquality of the gradation printing among the embodiments of the thermalprinthead of the invention.

The heat transfer ink sheet used in the described embodiments makes useof a heat-meltable ink as the material of the color layer. However, itwill be clear to those skilled in that art that the invention can beequally applied to the case where a sublimation type dyes, whichinherently permits a concentration gradation printing, are used as thecolor layer material. In such a case, a higher degree of gradation canbe attained by the multiplication of the concentration gradation effectafforded inherently by the sublimation type dyes and the dot areagradation effect offered by the invention.

What is claimed is:
 1. A gradation thermal printhead comprising aplurality of heat generating elements arranged in a line, each of saidheat generating elements having a pair of electrodes and a heatgenerating body connected between said electrodes, said heat generatingbody having an electric resistance value distribution such that theresistance value is changed in a staggered manner at least in thedirection of connection to said electrodes.
 2. A gradation thermalprinthead according to claim 1, wherein said staggered manner ofvariation in the electric resistance value is formed by at least one ofnotches and apertures formed in said heat generating body.
 3. Agradation heat transfer printing apparatus comprising: a gradationthermal printhead having a plurality of heat generating elementsarranged in a line, each of said heat generating elements having a pairof electrodes and a heat generating body connected between saidelectrodes, said heat generating body having an electric resistancevalue distribution such that the resistance value is varied in astaggered manner at least in the direction of connection to saidelectrodes; and an energy controlling means for controlling the energyapplied to said thermal printhead; said apparatus employing a heattransfer ink sheet having a base film and a color material layer formedas a transfer medium on said base film; whereby the area of transfer ofsaid color material layer corresponding to one heat generating elementis varied through the control of the electric energy applied to saidthermal printhead by said energy controlling means, thereby attaining agradation printing of an image.
 4. A gradation heat transfer printingapparatus according to claim 3, wherein said staggered manner ofvariation in the electric resistance value is formed by at least one ofnotches and apertures formed in said heat generating body.